1. Field of the Invention
The present invention relates to an improved process for treating a gaseous stream containing nitrogen oxides and oxygen and, more especially, to the separation of nitrogen oxides from the gaseous effluent from a nitric acid synthesis facility, and to the use of such process for the purification of certain residual gases.
2. Description of the Prior Art
The synthesis of nitric acid via oxidation of ammonia with oxygen has long been known to this art. The initial reaction which is conducted at elevated temperature produces nitric oxide, NO, which oxide is then itself oxidized by oxygen at a lower temperature, to produce nitrogen peroxide, NO.sub.2, which in turn undergoes with water, in oxido-absorption columns, or towers, a dismutation reaction producing nitric acid and nitric oxide. The nitric oxide is in its turn re-oxidized by the remaining oxygen and the process continues with dismutation.
A conventional plant for the production of nitric acid typically comprises two, or preferably three plate absorption towers. In the second and third towers, however, the oxygen partial pressure and the partial pressures of the nitrogen oxides (NO and NO.sub.2) in the gas feed are such that the oxidation and absorption process slows down to such a point that it would be necessary to considerably increase the number of plates in the column in order to remove the nitrogen oxides which have not been converted into nitric acid and which are present in the residual gas stream. It is for this reason that, in order to avoid excessively high capital investment, the residual gases are discharged to atmosphere containing substantial amounts of nitrogen oxides, for example, between 1000 and 2000 vpm (volumes per million). Discharging gas in this manner suffers from two serious disadvantages: on the one hand, the discharged gases constitute a loss in production, which can be assessed at about 1% or more, and, on the other hand, they cause a serious problem as regards environmental pollution. In this respect, the different national laws which have been recently published require the residual or vent gases to contain substantially smaller amounts of nitrogen oxides (NO.sub.x). Thus, the French standard requires a level of less than 400 vpm while the U.S. standard requires a level less than 255 vpm.
And it is precisely for this reason that many processes have to date been proposed for reducing the NO.sub.x content in the residual gas flow, to permissible values. Certain of these processes are predicated upon use of a reaction for destroying the nitrogen oxides by means of ammonia, if need be in the presence of suitable catalyst, whereby the nitrogen oxides are reduced to nitrogen. Such processes consume a substantial amount of ammonia. Other processes entail absorption of the nitrogen oxides in a suitable solution, for example, an aqueous base solution, or a calcium hydroxide suspension, to yield nitrites, the same being by-products which are difficult to put to a useful purpose (see U.S. Pat. No. 3,034,853). Finally, other processes involve complementary oxidation of the nitrogen oxides contained in the gas stream by means of an oxidizing agent, if need be in the presence of a solid catalyst, and absorption of the oxidized gas by an aqueous solution in the form of nitric acid, thereby producing a solution of nitric acid, at a concentration of more than 70% (see U.S. Pat. No. 4,081,517).
It too is known for the nitrogen oxides in a gas stream to be absorbed by means of an aqueous solution of dilute nitric acid, thereby producing a solution of nitrous acid therein, and a gas stream which is purified in respect of NO.sub.x. The contacting solution is then regenerated by the reverse reaction of absorption, modifying temperature and pressure, or oxidizing HNO.sub.2 with oxygen in a separate apparatus.
When the NO.sub.x content is at low levels (less than 2500 vpm), corresponding to those levels present in the residual gas from a nitric acid plant, the balanced equation: EQU NO+NO.sub.2 +H.sub.2 O.revreaction.2HNO.sub.2
determines the production of a low level of concentration of HNO.sub.2 in the solution. Such concentration is always at a low level, typically less than 5.multidot.10.sup.-2 mole/liter, and it is at a maximum when the concentration with respect to HNO.sub.3 in the absorption solution is lower than 5 N. It is for this reason that the aforesaid process for the removal by absorption of the NO.sub.x in a residual gas stream emanating from oxido-absorption towers requires gas/liquid phase contactors having dimensions which are determined as a consequence thereof.
Moreover, in industrial practice, when using plate-type oxido-absorption columns, the basis for calculating the dimensions, etc., of such columns, for precipitation of nitric acid, is bottomed essentially on the following two equations: EQU NO+1/2O.sub.2 .revreaction.NO.sub.2 ( 1) EQU 3NO.sub.2 +H.sub.2 O.revreaction.2HNO.sub.3 +NO (2)
as hereinbefore mentioned. This results in the columns being of a design which is characterized by the following features:
(i) the number of plates in the columns is quite high, typically more than 20 in total;
(ii) the distance between the plates is relatively substantial (on the order of 0.7 to 1 meter); and
(iii) the height of the gas-liquid emulsion formed on each plate is relatively small, on the order of about 0.1 meter, in order to promote the oxidation of NO to NO.sub.2 in the gaseous phase at a slow rate. In this fashion, the ratio R between the volume of column between two successive plates, and the volume occupied by the gas-liquid emulsion between said successive plates, is advantageously higher than 7, and preferably is approximately 9.
The prior art has also suggested that, in order to increase the level of efficiency with respect to oxidation/absorption of the NO.sub.x, the distance between the first plates should be reduced and the distance between the last plates should be increased in consequence, with a constant number of plates, of the oxido-absorption volume and the liquid height, as per French Pat. No. 1,255,373. By conducting the operation according to this patent, in a 23-plate installation, the best result achieved is a reduction in the proportion of NO.sub.x in the residual gas, all other things being equal, from 2400 to 1200 vpm at typical operating pressure (on the order of 4 bars).
More characteristically in the prior art, with the gases having a NO.sub.x content of less than 6000 vpm and preferably less than 2500 vpm, the ratio K between the liquid retention volume in m.sup.3 in the corresponding section of the gas/liquid phase contactor, with the treated gas flow rate being in Nm.sup.3 /h, is lower than 3.5.multidot.10.sup.-4 hour, and preferably is approximately 2.5.multidot.10.sup.-4 hour.
Using an industrial unit designed as indicated above (R=9) and comprising fifty oxido-absorption plates, the present applicants have studied and determined the variation with respect to the partial pressures of nitrogen oxides above each plate, and the following conclusions have been reached:
(1) Absorption becomes progressively less and less efficient, as the columns are traversed; and
(2) The difference (1/P.sub.n)-(1/P.sub.n-1) in dependence upon the number of the plate, in which difference P.sub.n-1 represents the partial pressure of the NO.sub.x in the gas impinging onto the plate of row n and P.sub.n represents the partial pressure of the NO.sub.x of the gas exiting the plate of row n, becomes substantially constant as from the plate of row 20 of said industrial unit, this being the plate at the discharge of which the proportion of NO.sub.x in the gas is less than 6000 vpm and where operation is in the presence of countercurrently flowing dilute nitric acid, said flow being with respect to the gas.
This points to an overall second-order phenomenon with respect to the partial pressure of the NO.sub.x which are contained in the gas, and casts doubt on the very principle of manufacture and dimensioning of the columns for treating gases with a low NO.sub.x content. Consequently, and without wishing to be bound to any particular theory, when the partial pressure of the NO.sub.x in the gas is low (lower than 6000 vpm), the overall phenomenon of oxido-absorption in dilute solutions of nitric acid is limited by a reaction in the liquid phase. In such solutions, the soluble form of the NO.sub.x being HNO.sub.2, it would appear that it is the transformation of HNO.sub.2 into HNO.sub.3 which is the reaction that limits the total rate of flow of converted gas and not, as heretofore generally accepted, the gaseous phase oxidation reaction of NO into NO.sub.2. These findings are therefore somewhat contrary to the accepted hypotheses which govern the design of the oxido-absorption columns. Consequently, in order to improve the overall efficiency of oxido-absorption columns and, more generally, a countercurrent oxido-absorption apparatus, when the gas has a low content of NO.sub.x, it is therefore necessary to promote the conversion of the HNO.sub.2 in the liquid phase into HNO.sub.3.